Conventional domestic heating systems typically have an aquastat for controlling the firing of a boiler in response to a range of temperature sensed by a probe placed in the vicinity of the boiler jacket. The boiler is generally fired when the probe senses a low point of the temperature range, such as 160.degree. F., and shuts off when the probe senses a high temperature, such as 180.degree. F., regardless of the actual demand. For example, if hot water is needed for domestic uses, the boiler will fire at 160.degree. F., raise the temperature to 180.degree. F., and then shut off. The temperature of the water will drop off until it reaches 160.degree. F., at which time the boiler is refired. Such conventional systems require overcompensation in that they expend more heat than is necessary to heat water to meet demand at a desired temperature.
The location of the aquastat probe in the vicinity of the boiler results in repeated refiring of the boiler as the water temperature fluctuates within the given temperature range due to hot water demand. Even after hot water demand is terminated, the system may continue to fire without discrimination until the high temperature level is reached, which mitigates against efficient heating. Such systems must continuously regenerate hot water at the low point of the temperature range and continue firing until the high point is reached, regardless of the actual demand for hot water.